Measurements of carpellate aments provided in the identification key are from infructescences that are fully mature and beginning to disarticulate. Measurements (especially width) taken from before this stage of maturation will be too narrow and cause problems in the key.
Betula platyphylla Sukaczev has been attributed to
MA on the basis of a collection from along the banks of the Neponset River—
Svenson 11746 (
GH!). However, the specimen is clearly misidentified and has leaf blades that are of the wrong shape, are too large, and have too many pairs of lateral veins (among other discrepancies). Though the identity of the specimen is still in question, it is superficially similar to
B. papyrifera.

1b.
Leaf blades obscurely to evidently doubly serrate with pointed teeth (the teeth often blunt in
B. minor), ovate, rhombic, or triangular to narrow-ovate or oblong-ovate, acute to long-acuminate at the apex

5a.
Leaf blades triangular or rhombic-triangular to rhombic-ovate, acuminate to long-acuminate at apex
[Fig. 486], glabrous to sparsely pubescent along and in the axils of major veins; central lobe of carpellate scales much shorter than the lateral lobes

5b.
Leaf blades ovate to narrow-ovate or rhombic-ovate (to rhombic in
B. nigra), obtuse to acute or short-acuminate at the apex, usually sparsely to moderately pubescent along and in the axils of major veins; central lobe of carpellate scales as long as or longer than the lateral lobes

7b.
Leaves with broad-ovate to narrow-ovate blades (sometimes rhombic-ovate in
B. pubescens) and glabrous to pubescent, but not tomentose, petioles; carpellate aments 5–12 mm thick in fruit; body of samara as wide as or narrower than the individual wings